Wednesday, August 06, 2008

CCD, CMOS image sensor

CCD (charge coupled device) and CMOS (complementary metal oxide semiconductor) image sensors are two different technologies for capturing light and converting it into electrical signals. Both types of imagers convert light into electric charge and process it into electronic signals. In a CCD sensor, every pixel's charge is transferred through a very limited number of output nodes (often just one) to be converted to voltage, buffered, and sent off-chip as an analog signal. All of the pixel can be devoted to light capture, and the output's uniformity (a key factor in image quality) is high. In a CMOS sensor, each pixel has its own charge-to-voltage conversion, and the sensor often also includes amplifiers, noise-correction, and digitization circuits, so that the chip outputs digital bits.

Eight attributes characterize image sensor performance:

  1. responsivity: the amount of signal the sensor delivers per unit of input optical energy. CMOS imagers are superior to CCS.
  2. Dynamic range: the ratio of a pixel's saturation level to its signal threshold.
  3. Uniformity: the consistency of response for different pixels under identical illumination conditions. In CMOS, each pixel had an openloop output amplifier and the offset and gain of each amplifier vary, so it is worse than CCD
  4. Shuttering: the ability to start/stop exposure arbitrarily.
  5. Speed, COM has the advantage over CCS because all camera functions can be placed on the image sensor.
  6. Windowing, one unique capability of CMOS to read out of portion for image sensor. It allows elevated frame or line rates for small ROI.
  7. antiblooming: the ability to gracefully drain local over-exposure without compromising the rest of the image in the sensor. CMOS has natural blooming immunity. CCD requires specific engineering to achieve this.
  8. Biasing and clocking.

CMOS sensor has an advantage because all circuit functions can be placed on a single integrated circuit chip.

An active-pixel sensor (APS),  is an image sensor consisting of an integrated circuit containing an array of pixel sensors, each pixel containing a photodetector and an active amplifier.

Digital color cameras generally use a Bayer mask over the CCD. Each square of four pixels has one filtered red, one blue, and two green (the human eye is more sensitive to green than either red or blue).

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